This invention relates to an antenna and receiver input system for discriminating among signals propagated via multiple paths from a transmitter; and more particularly to such a system for use in a microwave landing system.
Various forms of radar and radio communication have been used in the past with voice and instruments to provide aids to landing of aircraft, particularly when visibility is poor because of fog or other weather conditions. For several years there has been standardization on the Instrument Landing System (ILS). To improve performance and provide truly all weather operation, a Microwave Landing System (MLS) has been proposed. A good illustration of the concept and characteristics of the MLS and a comparison to the ILS is found in National Geographic (Vol. 152, No. 2) August 1977 at pages 230 and 231.
Further background information can be found in "Electromagnetic Waves and Radiating Systems" by E. C. Jordan and K. G. Balman, Prentice-Hall, Englewood Cliffs, N. J. 1968, particularly in Chapter 11, Chapter 14, pages 143-148, and pages 345-347; in "The Theory of Linear Antennas" by R. W. P. King, Harvard University Press, Cambridge, Mass., 1956, particularly in Chapter III and at page 309; and in "Reference Data for Radio Engineers", H. P. Westman (Editor), Howard W. Sams, Inc., New York, N. Y. 1972, pages 6-1, 4. All of the above publications are hereby incorporated herein by reference.
Guidance errors can be introduced into microwave landing systems from false signals that are received from energy reflected by objects present along runways. Studies have shown that the use of horizontally polarized radiation, in contrast to vertically polarized radiation, reduces the amplitudes of the false signals because of its lower reflection coefficient for vertical objects at small angles of incidence.